Use and fabrication of microscaffolds and nanoscaffolds

ABSTRACT

A scaffold includes struts that intersect at nodes. In some instances, a cross section of the cores has at least one dimension less than 100 microns. The core can be a solid, liquid or a gas. In some instances, one or more shell layers are positioned on the core.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/254,824, filed Apr. 16, 2014, which application claims the benefit ofU.S. Provisional Patent Application 61/817,633, filed on Apr. 30, 2013;and also claims the benefit of U.S. Provisional Patent Application61/817,637, filed on Apr. 30, 2013; and also claims the benefit of U.S.Provisional Patent Application 61/812,621, filed on Apr. 16, 2013; andalso claims the benefit of U.S. Provisional Patent Application61/812,633, filed on Apr. 16, 2013; and also claims the benefit of U.S.Provisional Patent Application 61/938,503, filed on Feb. 11, 2014; andis a continuation of PCT Patent Application number PCT/US2014/034421,filed on Apr. 16, 2014, each of which is incorporated herein in itsentirety for any and all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Grant No.DGE1144469 and under Grant No. DMR1204864 awarded by the NationalScience Foundation. The government has certain rights in the invention.

FIELD

The present invention relates to scaffolds and more particularly tomicroscaffolds and nanoscaffolds.

BACKGROUND

Biological materials such as wood, bone, and crustaceous shells havedesirable mechanical properties such as high damage tolerance and lowdensity. Attempts have been made to mimic the structure of thesebiological materials in order to capture these mechanical properties.However, these biological materials have gained these advantages bycombining material selection with structural arrangements havingnanoscale features. The ability to fabricate structures with the desireddimensions and materials has been limited by the availablenanofabrication technologies. As a result, there is a need forfabrication technologies that permit use of these materials onstructures having nanoscale features.

SUMMARY

A scaffold includes struts that intersect at nodes. Each of the strutshas a core. A cross section of the cores has at least one dimension lessthan 10 microns where the cross section is taken perpendicular to alongitudinal axis of the core. The core can be a solid, liquid or a gas.In some instances, one or more shell layers are positioned on the core.

Another embodiment of the scaffold includes struts intersecting atnodes. The struts that terminate at two nodes each has a length equal tothe distance between the two nodes at which the strut terminates. Atleast a portion of the struts that terminate at two nodes each has alength less than 100 microns.

Another embodiment of the scaffold includes struts intersecting atnodes. At least a portion of the struts intersect so as to defineperiodically spaced unit cells in the scaffold. The unit cells arerepeated with a period length less than 200 microns, or less than 100microns.

A method of fabricating a scaffold includes employing multiphotonabsorption to define frame members in a frame precursor. The method alsoincludes using the frame members to form struts that intersect at nodes.

In some instances, the scaffold serves as an electrode or is included inan electrode. In some instances, the scaffold serves as an electrode oris included in an electrode of a functioning device. Examples offunctioning devices include, but are not limited to, batteries, fuelcells, capacitors, and supercapacitors.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A and FIG. 1B illustrates a portion of a scaffold. A verysimplified scaffold construction using a cubic unit cell is shown inorder to simplify the drawings. The scaffold includes struts thatintersect at nodes. FIG. 1A is a sideview of a portion of the scaffold.

FIG. 1B is a cross section of the scaffold.

FIG. 1C is a perspective view of a portion of a strut.

FIG. 1D is a cross section of a strut.

FIG. 1E is a cross section of a node where four struts intersect oneanother.

FIG. 2A through FIG. 2C illustrate another embodiment of a strut. FIG.2A is a perspective view of the strut.

FIG. 2B is a cross section of a strut constructed according to FIG. 2A.

FIG. 2C is a cross section of a node where four struts constructedaccording to FIG. 2A intersect one another.

FIG. 2D is a cross section of a strut having a shell that includesmultiple different shell layers.

FIG. 3 is a sideview of an octahedral unit cell.

FIG. 4A illustrates a two-dimensional kagome lattice.

FIG. 4B is a perspective view of a unit cell for a three-dimensionalkagome lattice.

FIG. 4C is a perspective view of a portion of a three-dimensional kagomelattice.

FIG. 5A illustrates struts arranged in a lattice of auxectic structures.

FIG. 5B illustrates a unit cell for a scaffold that includesintermediate struts extending between auxectic structures arranged asshown in FIG. 5A.

FIG. 6A through FIG. 6H illustrate a method of generating a scaffoldfrom a device precursor. FIG. 6A is a cross section of a deviceprecursor having a frame precursor on a substrate.

FIG. 6B is a cross section of the device precursor after a frame isformed in the frame precursor of FIG. 6A.

FIG. 6C is a cross section of a frame member shown in FIG. 6B takenalong the line labeled C in FIG. 6B.

FIG. 6D is a cross section of the device precursor after the remainingframe precursor is removed from the device precursor of FIG. 6B and FIG.6C.

FIG. 6E is a cross section of the device precursor after a shell isformed on frame members in the device precursor of FIG. 6D.

FIG. 6F is a cross section of a frame member shown in FIG. 6E takenalong the line labeled F in FIG. 6E.

FIG. 6G is a cross section of the device precursor after the framemembers are removed from the device precursor of FIG. 6E and FIG. 6F.

FIG. 6H is a cross section of a frame member shown in FIG. 6G takenalong the line labeled H in FIG. 6G.

FIG. 7A through FIG. 7J illustrate another embodiment of method forgenerating a scaffold having octahedral unit cells. FIG. 7A is a crosssection of a device precursor having a frame precursor on a substrate.

FIG. 7B is a cross section of the device precursor after frame membersare formed in the frame precursor of FIG. 7A.

FIG. 7C is a cross section of a frame member shown in FIG. 7B takenalong the line labeled C in FIG. 7B.

FIG. 7D is a cross section of the device precursor after the frame areremoved from the device precursor of FIG. 7B and FIG. 7C so as to formframe voids in the frame precursor.

FIG. 7E is a cross section of a frame void shown in FIG. 7D taken alongthe line labeled E in FIG. 7D.

FIG. 7F is a cross section of the device precursor after the frame voidsin the device precursor of FIG. 7D and FIG. 7E are filled.

FIG. 7G is a cross section of a frame void shown in FIG. 7F taken alongthe line labeled G in FIG. 7F.

FIG. 7H is a cross section of the device precursor after the remainingframe precursor is removed from the device precursor of FIG. 7F and FIG.7G.

FIG. 7I is a cross section of the device precursor.

FIG. 7J is a cross section of a frame member 44 shown in FIG. 7I takenalong the line labeled I in FIG. 7I.

FIG. 8A through FIG. 8C illustrate four struts intersecting at a nodewhere the struts are offset from one another. FIG. 8A is a sideview ofthe node.

FIG. 8B is a topview of the node.

FIG. 8C is the sideview illustrated in FIG. 8A after a force has beenapplied to the node in the direction of the arrow labeled F in FIG. 8A.

FIG. 9 is a schematic of a typical battery construction.

DETAILED DESCRIPTION

As used herein and in the appended claims, the singular forms “a,” “an,”and “the” include plural referents unless the context clearly dictatesotherwise. Thus, for example, reference to “a scaffold” includes aplurality of such scaffolds and reference to “the core” includesreference to one or more cores, and so forth.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to one of ordinary skill inthe art to which this disclosure belongs. Although methods and materialssimilar or equivalent to those described herein can be used in thepractice of the disclosed methods and compositions, the exemplarymethods, devices and materials are described herein.

Also, the use of “or” means “and/or” unless stated otherwise. Similarly,“comprise,” “comprises,” “comprising” “include,” “includes,” and“including” are interchangeable and not intended to be limiting.

It is to be further understood that where descriptions of variousembodiments use the term “comprising,” those skilled in the art wouldunderstand that in some specific instances, an embodiment can bealternatively described using language “consisting essentially of” or“consisting of.”

Any publications discussed above and throughout the text are providedsolely for their disclosure prior to the filing date of the presentapplication. Nothing herein is to be construed as an admission that theinventors are not entitled to antedate such disclosure by virtue ofprior disclosure.

Scaffolds have struts that intersect one another at nodes. Scaffoldfabrication methods can make use of multiphoton absorption to form thesescaffolds at high resolution levels. As a result, the scaffolds can beformed with surprisingly small features. For instance, the disclosedfabrication methods permit formation of scaffolds with microscale andnanoscale features that could not be achieved with the prior fabricationmethods that are generally limited to features above 50 microns.Further, multiphoton absorption allows scaffold features to be formed inthe center of the photoresist without the feature extending to theperimeter of the photoresist. Almost any scaffold configuration ispossible. For instance, the struts can be straight or curved and/ordifferent struts can have different dimensions. Further, complexscaffold configurations such as kagome lattices and auxectic structuresare possible. Further, the small dimensions that can be achieved permitcomplex strut intersections. For instance, two or more intersectingstruts can be offset relative to one another. Finally, the scaffoldfabrication techniques permit a broad range of materials to be includedin the scaffolds in a broad range of configurations. As a result, thescaffolds can provide a low weight and highly damage resistantmaterials. Further, these scaffolds provide an effective platform forstudying biological structures such as wood, bone, and crustaceousshells.

The use of these scaffolds is not limited to low weight materials thatare resistant to physical damage. For instance, one suitable applicationof these scaffolds is electrodes in applications such aselectrochemistry, electrophoresis, capacitors, photovoltaics, micro- andnano-electrochemical systems (MEMS and NEMS), optoelectronic devices,catalysis, and lab-on-a-chip. For instance, the ability to use a broadrange of materials with these scaffolds allows the struts of thescaffold to include or consist of the active material for a batteryelectrode. A challenge with high capacity active materials for batteriesis the extensive volume changes during that these active materialsexperience during charging and/or discharging of the battery. Theexpansion and contraction of the active material can serve as a sourceof electrode damage and/or battery failure. When a battery electrode isconstructed such that struts of the scaffold include or consist of theseactive materials, the struts can swell into the spaces between thestruts during expansion of the active materials. Additionally, thestructure of the scaffold can spread the volume expansion throughout theelectrode. As a result, the tolerance of the electrode for the expansionof the active materials is enhanced. The enhanced tolerance to expansioncan make it possible for the batteries to use materials that were notpreviously practical. For instance, silicon is desirable for use as anactive material in secondary lithium ion batteries due to its highcapacity; however, silicon can swell as much as 400% duringintercalation of lithium ions into the silicon. This degree of swellinghas made the use of silicon impractical. However, the ability of thedisclosed scaffolds to accommodate this degree of swelling can permitthe use of silicon in batteries.

FIG. 1A and FIG. 1B illustrates a portion of a microstructure thatincludes a scaffold 10. FIG. 1A is a perspective view of a portion ofthe scaffold 10. FIG. 1B is a sideview of the scaffold 10. A verysimplified scaffold construction using cubic cells is shown in order tosimplify the drawings. More complex scaffold structures are disclosedbelow.

The scaffold includes or consists of struts 14. FIG. 1C is a perspectiveview of a portion of a strut 14 and FIG. 1D is a cross section of astrut 14. The cross section of FIG. 1D is taken perpendicular to thelongitudinal axis of the strut 14 and can represent the cross section ofthe scaffold shown in FIG. 1A taken along the line labeled E. The strut14 includes a solid core 16. The struts 14 intersect one another atnodes 16. FIG. 1E is a cross section of a node 16 where four struts 14intersect one another. The materials of the different struts 14 arecontinuous with one another. Other node 16 constructions are possible.For instance, there can be interfaces between different struts 14 and/orbetween different materials from different struts 14. Suitable materialsfor the core 16 include, but are not limited to, metals, alloys, andpolymers.

FIG. 2A through FIG. 2C illustrate another possible construction for thestruts 14. FIG. 2A is a perspective view of a portion of a strut 14 andFIG. 2B is a cross section of a strut 14. The cross section of FIG. 2Bcould represent the cross section of the scaffold shown in FIG. 1A takenalong the line labeled E. The struts 14 include a shell 20 on the core16. The core 16 can be a solid, a liquid, or a gas. In some instances,the core 16 is filled with the ambient atmosphere in which the scaffoldis positioned. Since the core 16 can be a liquid or a gas, the core 16can be defined by an interior of the shell 20. For instance, an interiorsurface of the shell 20 can define a lumen that extends longitudinallythrough the strut 14. FIG. 2C is a cross section of a node 18 where fourstruts 14 intersect one another. The materials of the different struts14 are continuous with one another through the node 18. For instance,the cores 16 of the struts 14 are continuous with one another and/or theshells 20 of the struts 14 are continuous with one another. Other node18 constructions are possible. For instance, there can be an interfaceor interfaces between the materials of different cores 16 at the node 18and/or between different the materials from different shells 20.

Although FIG. 2A through FIG. 2C illustrate the shell 20 as beingconstructed of a single layer of material, the shell 20 can beconstructed of more than one shell layer 22 as illustrated in FIG. 2D.Each shell layer 22 includes or consists of one or more shell materials.Different shell layers 22 can have the same materials or differentmaterials. Materials for a shell layer include materials that can beformed on a solid core using deposition techniques. Examples ofdeposition techniques include vapor deposition techniques such aschemical vapor deposition (CVD) and its variants such as atomic layerdeposition (ALD). Examples of suitable materials for use as a shelllayer include, but are not limited to, metals, alloys, and ceramics suchas alumina. Although FIG. 2A through FIG. 2C illustrate the shell 20surrounding the core 16, the core 16 can include other arrangements ofmaterials positioned on the surface of the core 16. For instance, thecore 16 can include materials coated on the surface of the core 16 suchthat islands or aggregates of the materials are located on differentregions of the core 16 surface.

A first dimension of the core 16 is labeled D in FIG. 1C and FIG. 2A.The illustrated cores 16 have elliptical cross sections. Accordingly,the illustrated cores have a minor axis and a major axis. The firstdimension represents the smallest dimension of the cross section thatmust be defined by the scaffold fabrication technique. Accordingly, thefirst dimension represents the length of the minor axis. The crosssection of the core 16 can have other geometries including, but notlimited to, round, square, rectangle, oval, and a plus sign. As aresult, the first dimension can represent the width, diameter, lengthand/or width of a branch on a plus sign. The first dimension can be lessthan 50 microns, 10 microns, 1 micron, 500 nm, or 150 nm. The strutsalso have a length. When a strut terminates at two nodes, the length ofthe strut is the distance that the strut extends between nodes. Thedisclosed method of fabricating the scaffold also permits the struts tobe fabricated with lengths having these dimensions. As a result, in someinstances, at least a portion of the struts in a scaffold have a lengthless than less than 1000 microns, 50 microns, 10 microns, 1 microns, 500nm, or even 150 nm. Accordingly, at least a portion of the struts in ascaffold can have at least one dimension that is less than 50 microns,10 microns, 1 micron, 500 nm, or even 150 nm.

In FIG. 2A, a second dimension of the strut 14 is labeled T and a thirddimension is labeled W. The second dimension describes a thickness of ashell layer 22 in that it describes a distance between an outer surfaceof the shell layer 22 and the inner surface of the shell layer 22. Insome instances, at least a portion of the struts have a second dimensionless than 1 micron, 200 nm, 100 nm, or 50 nm and/or greater or equal to1 angstrom. The third dimension is an external dimension in that itdescribes a distance between two locations on an outside of the strut14. For instance, the third dimension can describe a distance betweentwo different locations on the outer surface of the shell 20. Theillustrated strut 14 has a round cross section. Accordingly, the thirddimension represents the diameter of the strut 14. However, the crosssection of the strut 14 can have other geometries including, but notlimited to, a square rectangle, oval, ellipse, and a plus sign. As aresult, the third dimension can represent the width, diameter, length ofa major axis, length of a minor axis, length and/or width of a branch ona plus sign. Even when the struts 14 have a shell 20, the smalldimensions of the core 16 allow the third dimension to be reducedrelative to prior scaffolds. In some instances, at least a portion ofthe struts have a third dimension less than 50 microns, 10 microns, 1microns, 500 nm, or even 150 nm.

In one example, the struts have a structural ratio (seconddimension/third dimension) that is less than or equal to 0.02. In aparticular example, the struts have an elliptical cross section and aratio of the shell thickness to the major axis length is less than orequal to 0.02. In another example, the struts have an elliptical crosssection, are hollow with a ceramic shell, and a ratio of the shellthickness to the major axis length is less than or equal to 0.02. Inanother example, the struts have an elliptical cross section, are hollowwith an alumina shell, and a ratio of the shell thickness to the majoraxis length is less than or equal to 0.02. In some instances, thescaffold has a density less than 10 mg/cm³.

The disclosed methods for fabricating the scaffolds permit one or morevariables selected from the group consisting of the first dimension, thesecond dimension, and the third dimension to be varied over a widerange. As a result, the porosity of the scaffold can be tightlycontrolled and surprisingly low porosity levels can be achieved. In someinstances, one or more of these variables is varied so as to achieve aporosity less than 25%, or 10%.

The disclosed fabrication method permits the use of more complexscaffolds and/or unit cells. For instance, the struts 14 can be arrangedin octets or octahedrons as illustrated in FIG. 3 . The unit cell is thesmallest unit that can be repeated so as to create the portion of ascaffold that is periodic. As a result, when the octahedron includes twofour sided pyramids of the same dimensions, the four sided pyramidserves as the unit cell. Accordingly, the scaffold can include orconsist of repeated four sided pyramids or repeated octahedrons.

Another suitable scaffold of interest includes the struts 14 arrangedsuch that the scaffold includes or consists of a kagome lattice. FIG. 4Aillustrates struts arranged in a two-dimensional kagome lattice.Although the illustrated lattice is called “two-dimensional,” thelattice need not be planar. For instance, the nodes and/or struts neednot be co-planar. Each node 18 of the two-dimensional kagome latticeincludes two triangles alternating 26 with two hexagons 28.

FIG. 4B is a perspective view of a unit cell suitable for constructing athree-dimensional kagome lattice. FIG. 4C illustrates a portion of ascaffold having the struts are arranged in a three-dimensional kagomelattice using the unit cells of FIG. 4B. The surprising ability to formkagome lattices in scaffolds having these dimensions can increase thefracture resistance of the scaffold due to crack tip blunting. Thescaffold can include kagome lattices without being arranged asillustrated in FIG. 4B and FIG. 4C. For instance, the scaffold caninclude struts that extend between two-dimensional kagome latticesconstructed as shown in FIG. 4A. Accordingly, the scaffold can include athree-dimensional kagome lattice or can include multiple two-dimensionalkagome lattices. As a result, in some instances, the nodes of thescaffold includes at least two triangles alternating 26 with at leasttwo hexagons 28.

The scaffold can also be configured such that the unit cells have anegative Poisson's ratio. For example, the unit cells can be constructedsuch that when compressed by an applied force, the unit cells becomethinner perpendicular to the applied force. In one example, the unitcells are constructed so as to include one or more auxectic structures32. For instance, FIG. 5A illustrates struts 14 arranged in a lattice ofauxectic structures. A scaffold can have intermediate struts 14extending between adjacent lattices that each includes the auxecticstructures. For instance, FIG. 5B illustrates a unit cell for a scaffoldthat includes intermediate struts 14 extending between auxecticstructures arranged as shown in FIG. 5A. Only the struts 14 positionedat the front of the unit cell are shown in order to reduce thecomplexity of the drawing. The intermediate struts 14 are also arrangedso as to form auxectic structures although other arrangements for theintermediate struts 14 are possible. The auxectic structures of FIG. 5Aand FIG. 5B are exemplary and a scaffold can include other auxecticstructures.

The scaffold need not be periodic; however, as is evident from FIG. 1Athrough FIG. 5B, the disclosed fabrication techniques can optionally beused to create scaffolds where all or a portion of the scaffold isperiodic. For instance, the struts can optionally be arranged so as todefine multiple unit cells that are repeated periodically to form all ora portion of the scaffold. Accordingly, all or a portion of the scaffoldcan optionally be periodic. For instance, the portion of the scaffoldillustrated in FIG. 1A includes multiple cubic unit cells that arerepeated so as to create the illustrated portion of the scaffold 10.Lateral repetition of the unit cells defines a cell layer 12 in thescaffold. Multiple cells layers are stacked to add height to thescaffold. In this configuration, the scaffold includes a first periodlabeled P₁ in FIG. 1B and a second period labeled P₂ in FIG. 1B. Thefirst period is the distance between the centers of the unit cells unitcells in the same layer and the second period is the shortest distancebetween the center of a unit cell in one layer and the center of a unitcell in another layer. A suitable length for the first period and/or thesecond period include lengths less than 200 microns, 50 microns, 10microns, 1 micron, or even 500 nm and/or greater than 50 nm. The celllayers 12 can be repeated so as to provide more than 2 or 3 cellslayers.

FIG. 6A through FIG. 6H illustrate a method of generating a scaffoldhaving octahedral unit cells. A frame precursor 40 is formed on asubstrate 42 so as to provide a device precursor having a cross sectionas shown in FIG. 6A. FIG. 6A is a cross section of the device precursor.The frame precursor 40 can be a negative photoresist. In some instances,the photoresist includes or consists of a photopolymer. Photopolymerschange their chemical properties when exposed to light or to light ofcertain wavelengths. In some instances, the photopolymer changes itssolubility in a lithography developer in response to exposure of thephotopolymer to light of a particular wavelength or range ofwavelengths. For instance, a suitable photopolymer can polymerize and/orcross link in response to exposure of the photopolymer to the light. Anexample of a suitable photopolymer includes, but is not limited to,IP-DIP 780 photoresist available from Nanoscribe, Inc. Suitablematerials for the substrates 42, include, but are not limited to, metalssuch as aluminum.

A frame is formed in the frame precursor 40 of FIG. 6A so as to form thedevice precursor of FIG. 6B and FIG. 6C. FIG. 6B is a cross section ofthe device precursor. The frame includes multiple frame members 44 thatintersect at nodes. FIG. 6C is a cross section of a frame member 44shown in FIG. 6B taken along the line labeled C in FIG. 6B. As willbecome evident below, the frame members 44 become the core of the strutsor are replaced by one or more materials that serve as the cores for thestruts in the final scaffold. Accordingly, the arrangement of the framemembers 44 is the same as or approximates the desired arrangement forthe cores of the struts in the resulting scaffold. As a result, in someinstances, the frame members 44 have the same dimensions as are setforth for the cores of the struts.

A suitable method for forming the frame in the frame precursor 40includes, but is not limited to, multiphoton photolithography such astwo-photon lithography. Other names for multiphoton photolithographyinclude direct laser writing and direct laser lithography. Inmultiphoton photolithography, the frame precursor 40 is transparent orsubstantially transparent to the wavelength of a light source so as tosuppress single photon absorption relative to multiphoton absorption.The multiphoton absorption can cause the desired chemical change of theframe precursor 40. For instance, when the frame precursor 40 includesor consists of a photopolymer, the multiphoton absorption can causepolymerization of the photopolymer and/or cross-linking of thephotopolymer. When the photopolymer is IP-DIP 780 photoresist, themultiphoton absorption can cross-link the polymer.

In some instances, the light source used for multiphoton absorption isconfigured to have a focal point. In some instances, the light intensityrequirements needed for multiphoton absorption cause the desiredchemical change to occur at the focal point or focal volume of the lightsource without substantially occurring outside of the focal volume. Bycontrolling the location of the focal point or focal volume of the lightsource within the frame precursor 40, the location of the chemicalchange within the frame precursor 40 can be controlled. For instance,the relative positions of the device precursor and light source can bechanged such that the focal point or focal volume of the light sourceeffectively scans the desired locations of the frame members 44 withinthe frame precursor 40. Since the desired chemical change is localizedrelative to the focal point or focal volume of the light source, the useof multiphoton absorption permits the scaffold features to be formedwith the above dimensions. Further, since the desired chemical change islocalized to the focal point, the chemical change does not substantiallyoccur between the light source and the focal point. Accordingly, a traceof the chemical change does not occur between the target location of theframe member 44 and the light source. As a result, features can beformed centrally within the frame precursor 40 without the featureextending to the perimeter of the frame precursor 40. The ability toform features centrally within the frame precursor 40 permits theformation of frame members 44 and the resulting struts in nearly anyconfiguration.

The remaining frame precursor 40 can be removed from the deviceprecursor of FIG. 6B and FIG. 6C so as to form the device precursor ofFIG. 6D. For instance, when the frame precursor 40 acts as a negativephotoresist, the frame precursor 40 can removed with a developer. Ininstances where a shell 20 is not desired on the core of the struts andthe frame members 44 are constructed of the material that is desired forthe cores of the struts, the frame on the device precursor of FIG. 6Dcan serve as the scaffold.

When it is desirable for the core to include a shell 20, the shell 20can be formed on frame members 44 in the device precursor of FIG. 6D soas to form the device precursor of FIG. 6E and FIG. 6F. FIG. 6E is across section of the device precursor. FIG. 6F is a cross section of aframe member 44 shown in FIG. 6E taken along the line labeled F in FIG.6E. Suitable methods of forming each shell layer include, but are notlimited to, deposition and/or growth techniques such as chemical vapordeposition (CVD), atomic layer deposition (ALD). Since atomic layerdeposition (ALD) can provide a coating with a thickness at the angstromlevel, atomic layer deposition (ALD) is an example of a method that issuitable for achieving shell layers having the thickness levelsdescribed above. Atomic layer deposition (ALD) typically includessequentially reacting different gas phase precursors with the surface ofthe frame members 44 in a self-limiting chemical process. In someinstances, the process is repeated in order to provide the shell layerwith the desired thickness. Different shell layers can be formed usingdifferent deposition techniques or using the same deposition technique.In instances where the frame members 44 are constructed of the materialsthat are desired for the cores of the struts, the frame on the deviceprecursor of FIG. 6E and FIG. 6F can serve as the scaffold.

When it is desirable for the core to include a material that isdifferent from the material of the fame members, the frame members 44can be removed so as to form the device precursor of FIG. 6G and FIG.6H. FIG. 6G is a cross section of the device precursor. FIG. 6H is across section of a frame member 44 shown in FIG. 6G taken along the linelabeled H in FIG. 6G. Suitable methods for removing the frame members 44include, but are not limited to, etching. For instance, when the frameprecursor 40 is IP-DIP 780 photoresist, the frame members 44 can beremoved by oxygen plasma etching. When the shell 20 protects theunderlying frame members 44 from etching, it may be necessary to exposethe frame members 44 at one or more locations prior to etching. Suitablemethods of exposing the frame members 44 to prepare for etching include,but are not limited to, etching methods including Focused Ion Beam (FIB)etching.

Removal of the frame members 44 leaves a hollow shell 20 that canoptionally serve as the struts 14 in the final scaffold. Accordingly,removing the frame members 44 can form or leave the lumen 48 in theshell 20. The material that is currently in the lumen or that willoccupy the lumen at a later time can serve as the core of the struts. Insome instances, the interiors of the lumens are exposed to theatmosphere in which the scaffold is positioned. As the result, the cores16 of the struts 14 can be filled with a gas or a liquid depending onthe content of the atmosphere in which the scaffold is positioned. As anexample, when the device is located in air, air generally will occupythe core 16 of the struts 14 and when the device is located in a liquid,the liquid can occupy the core 16 of the struts 14. Accordingly, thematerial in the core 16 of the struts 14 can change as the scaffold ismoved from one location to another. Alternately, the device and/or theresulting scaffold can be encapsulated in a solid material and the core16 of the struts 14 can be filled with the solid encapsulating material.

In some instances, it may be desirable to fill the lumens 48 with asolid material so as to provide solid cores 16. Suitable methods forfilling the shells 20 include, but are not limited to, electroplating.As an example, when it is desirable to fill the hollow shells 20 with ametal, the substrate 42 can be electrically conducting and in contactwith the interior of the shell 20, the substrate 42 can be used toelectroplate at least the interior of the shell 20. As another example,deposition and/or growth techniques can be used to form another layer ofmaterial on the interior and/or exterior of the shell 20. Suitabledeposition or growth techniques include, but are not limited to,chemical vapor deposition (CVD), atomic layer deposition (ALD).

One or more shell layers can be treated as a sacrificial shell layer(not illustrated) during or after the removal of the frame members 44.The sacrificial shell layer can be the first shell layer formed on theframe members 44 or can be formed after other shell layers are formed onthe frame members 44. The sacrificial shell layer can be removed duringor after the removal of the frame members 44. Accordingly, additionalshell layers can be formed on a sacrificial shall layer before or afterthe removal of the frame members 44. A sacrificial shell layer can actsas a buffer between materials and/or can be used to increase thecompatibility between different materials during the fabricationprocess. In one example, a sacrificial shell layer is formed directly onthe frame members 44 and then the frame members 44 are removed. A secondshell layer is formed on the sacrificial shell layer and the sacrificialshell layer is then removed leaving the second shell layer to serve asthe struts. A suitable method for removing the sacrificial shell layerincludes, but is not limited to, etching. The sequence of forming ashell layer or a second sacrificial shell layer over a first sacrificialshell layer followed by removal of the first sacrificial shell layer canbe repeated.

FIG. 7A through FIG. 7J illustrate another embodiment of method forgenerating a scaffold having octahedral unit cells. A frame precursor 40is formed on a substrate 42 so as to provide a device precursor having across section as shown in FIG. 7A. FIG. 7A is a cross section of thedevice precursor. The frame precursor 40 can be a positive photoresist.In some instances, the photoresist includes or consists of aphotopolymer. Photopolymers change their chemical properties whenexposed to light. In some instances, the photopolymer changes itssolubility in a lithography developer in response to exposure of thephotopolymer to light of a particular wavelength or range ofwavelengths. For instance, a suitable photopolymer can polymerize and/orcross link in response to exposure of the photopolymer to the light. Anexample of a suitable photopolymer includes, but is not limited to,AZ4620 available from Microchem Corp. located in Newton, Mass. Suitablematerials for the substrates 42, include, but are not limited to, metalssuch as aluminum and electrically insulating materials.

Frame members 44 are formed in the frame precursor 40 of FIG. 7A so asto form the device precursor of FIG. 7B and FIG. 7C. FIG. 7B is a crosssection of the device precursor. The frame precursor 40 includesmultiple frame members 44. FIG. 7C is a cross section of a frame member44 shown in FIG. 7B taken along the line labeled C in FIG. 7B. As willbecome evident below, the frame members 44 will be replaced by one ormore materials that serve as the cores for the struts in the finalscaffold. Accordingly, the arrangement of the frame members 44approximates the desired arrangement for the cores of the struts in theresulting scaffold. As a result, in some instances, the frame members 44have the same dimensions as are set forth for the cores of the struts.

A suitable method for forming the frame members 44 in the frameprecursor 40 includes, but is not limited to, multiphotonphotolithography as described above. As noted above, multiphotonphotolithography can be performed using a focused light source andwithout a photomask. By controlling the location of the focal point orfocal volume of the light source within the frame precursor 40, thelocation of the chemical change within the frame precursor 40 can becontrolled. For instance, the device precursor can be moved relative tothe focal point of the light source such that the focal point or focalvolume of the light source effectively scans the desired locations ofthe frame members 44 within the frame precursor 40. In some instances,the locations where the scanning has caused the desired chemical changein the frame precursor 40 serve as the frame members 44.

The frame members 44 can be removed from the device precursor of FIG. 7Band FIG. 7C so as to form the device precursor of FIG. 7D and FIG. 7E.FIG. 7D is a cross section of the device precursor. FIG. 7E is a crosssection of a frame void 46 shown in FIG. 7D taken along the line labeledE in FIG. 7D. The frame members 44 can be removed by developing and/oretching. For instance, when the frame precursor 40 is AZ4620, the framemembers 44 can be removed by developing and oxygen plasma etching. Theremoval of the frame members 44 leaves frame voids 46 in the frameprecursor 40. As will become evident below, the remaining frameprecursor 40 acts as a template for the scaffold.

The frame voids 46 in the device precursor of FIG. 7D and FIG. 7E arefully or partially filled with a material so as to provide the deviceprecursor of FIG. 7F and FIG. 7G. FIG. 7F is a cross section of thedevice precursor. FIG. 7G is a cross section of a frame void 46 shown inFIG. 7F taken along the line labeled G in FIG. 7F. The material in thefame voids replaces the frame members 44 and can serve as cores 16 ofthe struts 14 in the resulting scaffold. Suitable methods for fillingthe frame voids 46 include, but are not limited to, electroplating. Asan example, when it is desirable to fill the frame voids 46 with ametal, the substrate 42 can be electrically conducting and the substrate42 can be used to electroplate the interiors of the frame voids 46.

The remaining frame precursor 40 can be removed from the deviceprecursor of FIG. 7F and FIG. 7G so as to form the device precursor ofFIG. 7H. FIG. 7H is a cross section of the device precursor. When theremaining frame precursor 40 acts as a photoresist, the frame precursor40 can be removed with a developer. In instances where a shell 20 is notdesired on the core 16 of the struts 14, the frame on the deviceprecursor of FIG. 7H can serve as the scaffold.

When it is desirable for the struts to include a shell 20, the shell 20can be formed on cores 16 in the device precursor of FIG. 7H so as toform the device precursor of FIG. 7I and FIG. 7J. FIG. 7I is a crosssection of the device precursor. FIG. 7J is a cross section of a framemember 44 shown in FIG. 7I taken along the line labeled J in FIG. 7I.Suitable methods of forming each shell layer include, but are notlimited to, deposition and/or growth techniques such as chemical vapordeposition (CVD) and its variants such as atomic layer deposition (ALD).In particular, atomic layer deposition (ALD) is suitable for achievingshell layers having the thickness levels described above. Atomic layerdeposition (ALD) typically includes sequentially reacting different gasphase precursors with the surface of the frame members 44 in aself-limiting chemical process. In some instances, the process isrepeated in order to provide the shell layer with the desired thickness.Different shell layers can be formed using different depositiontechniques or using the same deposition technique. In instances wherethe frame members 44 are constructed of the material that is desired forthe cores 16 of the struts 14, the frame on the device precursor of 7Hand FIG. 7I can serve as the scaffold.

In the methods of FIG. 6A through FIG. 7I, the substrate 42 canoptionally be removed after fabrication of the scaffold. Suitablemethods for removing the substrate 42 include, but are not limited to,etching and mechanical methods such as polishing. In some instances, theupper surface of the scaffold is bonded or attached to a secondsubstrate (not shown) before removal of the substrate 42. Accordingly,the substrate 42 can be used for fabrication of the scaffold without thesubstrate 42 being present in the final product. Additionally oralternately, the above scaffolds can optionally be encapsulated in anencapsulating material that fills in the voids in the scaffold and/orspans the gap between the struts in the scaffold. Suitable encapsulatingmaterials include, but are not limited to, polymers, epoxies.

The above methods of scaffold fabrication can employ multiphotonabsorption to form the frame members 44 in the frame precursor 40.

The above methods of scaffold fabrication allow features to be formedcentrally within the frame precursor 40. This ability allows thescaffolds to be formed with more complex features. For instance, all ora portion of the struts in a scaffold can be curved. These methods alsopermit a high level of resolution even at the nanometer and micron levelscaffold dimensions disclosed above. As a result, the scaffolds caninclude more sophisticated construction features. For instance, thestruts need not all have the same dimensions. Accordingly, in someinstances, a first portion of the struts have different cross sectionaldimensions than a second portion of the struts. For instance, the firstportion of the struts can have a thicker shell 20 and/or core 16 than asecond portion of the struts. Alternately or additionally, the firstportion of the struts can have larger cross sectional dimensions than asecond portion of the struts. Accordingly, struts that will experiencehigher forces during use of the scaffold can be designed to toleratehigher loads than other struts.

The level of detail that can be achieved with the above methods alsopermits more complex node 18 constructions. For instance, two or more ofthe struts that intersect at a node 18 can be offset relative to oneanother at the node 18. FIG. 8A through FIG. 8C illustrate four struts14 intersecting at a node 18 that is suitable for use in scaffoldshaving unit cells such as octets or octahedrons. FIG. 8A is aperspective view of the node 18. FIG. 8B is a topview of the node 18.The struts 14 have elliptical or substantially elliptical cross section.The struts 14 have a longitudinal axis that extend longitudinallythrough the center of the strut 14. This offset is best viewed from FIG.8B. The longitudinal axis for two of the struts 14 is illustrated inFIG. 8B. The struts 14 are offset such that the longitudinal axes of thestruts 14 do not intersect one another. Accordingly, a node center 64can be located between the longitudinal axes of offset struts 14 withoutany of the offset strut 14 longitudinal axes intersecting the center.The offset distance is labeled “Offset” in FIG. 8B. In some instances,the distance of the offset is more than 1 nm, 5 nm or 10 nm and/or lessthan 50 microns or less than 10 microns.

In FIG. 8A and FIG. 8B, each of the longitudinal axes passes on the sameside of the node center 64. For instance, each of the longitudinal axesis to the right of the node center 64. As a result, when a force isapplied in the direction of the arrow labeled F in FIG. 8A, the struts14 twist as shown in FIG. 8C rather that bending outwards. This twistingmay allow the struts 14 and/or node 18 to survive forces that could notbe tolerated if the struts 14 were aligned relative to one another.

FIG. 8A through FIG. 8C illustrate all of the struts 14 as being offsetrelative to one another; however, a node 18 can include struts 14 thatare offset and struts 14 that are not offset. For instance, in FIG. 8B,the struts 14 extending upward and downwards can remain offset while thestruts 14 extending to the left and right can be aligned with oneanother in that their longitudinal axes intersect within the node 18.Further, although the node 18 of FIG. 8A through FIG. 8C illustratesfour struts 14 intersecting at a node 18, the node 18 can include oddnumbers of struts 14 that are offset relative to one another.Accordingly, all or a portion of the nodes 18 in a scaffold can includetwo or more struts 14 that are offset relative to one another. When anode 18 includes two or more struts 14 that are offset relative to oneanother, the offset struts 14 can all be offset in the same directionrelative to the node center 64 but need not be offset in the samedirection relative to the node center 64. Accordingly, all or a portionof the nodes 18 in a scaffold can include two or more struts 14 that areoffset relative to one another where at least a portion of the strutsare offset in the same direction.

The above scaffolds can be applied in technologies such as biomedicaldevices, nanophotonics, and thermoelectrics. Additionally, thesescaffolds can be inserted into band gap driven technologies such asphotovoltaics and acoustic materials because the dimensions of thescaffold features can be tuned down at the nanometer level. A particularapplication of the above scaffolds is electrodes. The electrodes can beused in any application such as electrochemistry, electrophoresis,capacitors, photovoltaics, micro- and nano-eletrochemical systems (MEMSand NEMS), optoelectronic devices, catalysis, and lab-on-a-chip.

The scaffold itself can serve as the electrode. For instance, theelectrode can include or consist of a scaffold where the cores and/orthe shells each includes or consists of an electrically conductingmaterials such as a metal. Alternately, the scaffold combined with thesubstrate can serve as an electrode. For instance, the electrode caninclude or consist of both the scaffold and substrate where the coresand/or the shells each includes or consists of an electricallyconducting materials such as a metal. Accordingly, the electrode caninclude or consist of the scaffold or both the scaffold and thesubstrate. A liquid or solid sample in contact with the scaffold canpenetrate the regularly spaced openings between the struts. As a result,a high degree of contact between is the sample and the electrode isachieved and is evenly distributed across the electrode. Such electrodeconstructions are suitable for applications such as electrochemistry,electrophoresis.

In another application, the struts include one or more active materialsinto which a molecule (element, compound, ion or anion) intercalates ordeintercalates during operation of the electrode. For instance, thestruts can include or consist of a shell 20 that includes or consists ofone or more intercalation materials. One of the problems with electrodesused in applications where molecules intercalate into the electrode isswelling of the electrode. This swelling can degrade the performance ofthe electrode and/or damage the electrode. However, when the electrodeincludes one of the above scaffolds, the struts can swell into the spacebetween the struts and/or into the core. Accordingly, the scaffold canprovide an electrode with an enhanced tolerance to the intercalation ofthe molecule into the electrode during operation of the electrode.

Batteries are an example of an application where a molecule intercalatesand/or deintercalates from one or more electrodes. FIG. 9 presents aschematic of a typical battery construction. The battery includes apositive electrode 70, a negative electrode 72, a separator 74, and anelectrolyte 76 in a case 78. The separator 74 is positioned between thepositive electrode 70 and the negative electrode 72. The electrolyte 76activates the positive electrode 70 and the negative electrode 72. Thepositive electrode 70 and the negative electrode 72 each includes anactive medium 80 on a current collector 82. The active media include orconsist of the active materials that take part in the chemical reactionsduring the charging and/or discharge of the battery. Although notillustrated, the current collector 82 from the positive electrode 70 isin electrical communication with a positive terminal 84 and the currentcollector 82 from the negative electrode 72 is in electricalcommunication with a negative terminal 86.

The positive electrode 70 and/or the negative electrode 72 can includeone or more of the above scaffolds. For instance, the substrate 42 canbe electrically conducting and can accordingly serve as the currentcollector 82. The active medium 80 can include the scaffold. Forinstance, the shell 20 of the struts can include or consist of theactive material. In some instances, the cores of the struts are hollow.When the cores of the struts are hollow, the cores of the struts can besealed off from the atmosphere in which the scaffold is positioned orcan be exposed to the atmosphere in which the scaffold is positioned.For instance, openings made as a result of exposing the frame membersbefore removing the frame members can cause the cores of the struts tobe exposed to the atmosphere in which the scaffold is positioned.Additionally or alternately, the shell can be sufficiently porous topermit the atmosphere in which the scaffold is positioned to penetratethrough to the cores of the struts. Alternately, the cores can be sealedoff from the atmosphere in which the scaffold is positioned as a resultof process steps such as forming one or more shell layers and/or dummyshell layers on the struts subsequent to the exposure the frame membersbefore removing the frame members. When the cores of the struts arehollow, the electrolyte 76 is a liquid, and the cores of the struts areexposed to the atmosphere in which the scaffold is positioned, theliquid electrolyte 76 can fill the cores of the struts and can serve asthe cores of the struts. As a result, the electrolyte 76 can contact theinside and the outside of the struts and enhance the wetting of theactive medium 80 by the electrolyte 76. Alternately, the cores of thestruts can be filled with an electrically conducting medium such as ametal. The cores of the struts can be in contact with the substrate orcurrent collector 82 as is evident from the above discussion. As aresult, placement of the electrically conducting medium in the cores ofthe struts can provide an electrical pathway between the active medium80 in shell 20 and the current collector 82. The enhanced electricalcommunication between the active medium 80 and the current collector 82can accordingly reduce the internal resistance of the battery. In someinstances, the electrically conducting medium in the core of the strutsis the same material as the substrate 42. Suitable materials for use asa substrate 42 that will serve as a current collector 82 include, butare not limited to, metals such as copper, aluminum, and titanium.Suitable materials for use as the shell 20 that will include or consistof active material include, but are not limited to, silica.

In one example, the battery is a secondary (or rechargeable) lithium ionbattery and the negative electrode 72 includes a scaffold where thestruts have shells 20 that include or consist of silicon. In someinstances, the silicon is amorphous silicon due to the smaller volumechanges during intercalation as compared with crystalline silicon. Thesubstrate is an electrically conducting material that can act as acurrent collector. The cores can be hollow or filled with a solidmaterial. When the core is filled with a solid material, the solidmaterial can be an electrically conducting material that is inelectrical communication with the substrate. Additionally, the negativeelectrode 72 includes or consists of lithium cobalt oxide and/orgraphite as an active material. The electrolyte 76 can be any variety ofelectrolytes 76 such as traditional organic electrolytes 76 that includeone or more salts dissolved in an organic solvent. Additionally, theseparator 74 can include or consist of traditional separator materials.

Although the battery is disclosed in the context of a secondary battery,the battery can be a primary (non-rechargeable) battery. As a result,terms such as negative electrode also include anodes and positiveelectrode includes cathodes. Further, the battery can have a variety ofdifferent electrode configurations. For instance, the battery caninclude two or more electrodes wound in a jellyroll configuration or caninclude two or more electrodes in a stack configuration. As a result, abattery can include two or more electrodes that each includes one of thedisclosed scaffolds. Further, one or more of the electrodes can beconstructed without the current collector as is known in the batteryarts.

EXAMPLES Example 1

A scaffold was generated with a series of tessellated regular octahedralunits connected at their vertices. Each octahedron was made up of 7μm-long hollow struts with elliptical cross sections and wall thicknessof 75 nm. The resulting scaffold was 100 μm in each direction. TiNserves as the shell.

The method of FIG. 6A through FIG. 6H was used to generate the scaffold.IP-Dip 780 photoresist served as the frame precursor. Direct laserwriting (DLW) used two-photon lithography to form the frame memberswithin the frame precursor. The direct laser writing (DLW) was performedat a speed of 50 μm/sec and laser power of 10 mW using the PhotonicProfessional DLW system (Nanoscribe GmbH, Germany). The resulting framemembers were separated from the remaining frame precursor using adeveloper. The frame members were then conformally coated one monolayerat a time with TiN using an Oxford OpAL Atomic Layer Deposition (ALD)system (Oxfordshire, UK) at 140° C. The deposition was performed bysequentially cycling through the following steps: i) flowing thereactant dose of Titanium Tetrachloride (TiCl4) precursor for 30 ms, ii)purging the system for 5 sec, iii) plasma treatment with an N₂/H₂ gasmixture (25 sccm/25 sccm) for 10 sec, and iv) purging the system for anadditional 5 sec. This process was repeated until a 75 nm thick layerwas deposited. The TiN coating was then removed along an outer edge ofthe structure using focused ion beam (FIB) in the FEI Nova 200 Nanolabto expose a portion of the frame members. The frame members weresubsequently removed by etching in a barrel oxygen plasma etcher for 3hours under 100 W and 300 sccm oxygen flow. The remaining TiN served asthe struts of the scaffold.

In-situ compression experiments were performed on the octahedral unitcell by applying an axial load along the vertical axes of the unitcells. The experimentally obtained force vs. displacement data was inputinto a finite element method (FEM) framework to estimate the localstresses within the structure under the applied load. Results revealedthe attainment of very high von Mises stresses of 2.50 GPa, a valueclose to the theoretical strength of TiN without failure.

Example 2

An electrode is constructed using an aluminum substrate. An acrylicframe precursor was drop cast onto the substrate. Direct laser writing(DLW) used two-photon lithography to form the frame members within theframe precursor. The frame members had an elliptical cross section witha semi-major axis down to ˜300 nm and a semi-minor axis down to 75 nm.The undeveloped frame precursor was removed using propylene glycolmonomethyl ether acetate (PGMEA). Next a sacrificial shell layer ofsilica was deposited on the frame members using RF plasma sputtering.Focus ion beam milling was used to remove a portion of the silica so asto expose a portion of the frame members. The frame members weresubsequently removed by etching in a barrel oxygen plasma etcher for 3hours. A sub-100 nm thick layer of amorphous silicon was sputter-coatedconformally on the sacrificial shell layer. The sacrificial shell layerwas removed using buffered hydrofluoric acid. The remaining layer ofamorphous silicon served as the hollow struts of the scaffold. Thesilicon can serve as the active material of a battery electrode and thealuminum substrate can serve as the current collector.

Example 3

The method of FIG. 7A through FIG. 7J was used to generate a scaffold. A170 μm thick glass slide was obtained. A 60 nm thick layer of indiumtitanium oxide (ITO) was sputtered onto the glass slide so as to providean electrically conducting substrate. A positive photoresist of AZ4620(Microchem) was used as a frame precursor. The frame precursor was spunonto the substrate with a thickness of 20-30 μm and soft-baked for 3minutes at 110° C. Direct laser writing (DLW) used two-photonlithography to form the frame members within the frame precursor. Thedirect laser writing (DLW) was performed with a 780 nm wavelength laser.The frame members with cross sectional dimension of 900 nm to 4 micronscould be generated by changing the laser power from 0.5 mW to 1.2 mW.The frame members were removed from the frame precursor so as to leaveframe voids in the frame precursor. The frame members were removed bydeveloping the frame precursor in 1:4 diluted AZ400 (Microchem) foreight minutes followed by exposure to oxygen plasma at 100 W and 300sccm for two minutes. The resulting frame voids were opened all the waydown to the substrate. Potentiostatic electrodeposition was then used ina miniature three-electrode electrochemical cell in order to place thematerial for the cores of the struts in the voids. In one instance, theframe voids were filled with nickel. In these instances, theelectroplating was performed at 2V in an aqueous Ni bath containing 240g/l NiSO₄.6H₂O, 45 g/l NiCl₂.6H₂O, and 40 g/l H₃BO₃. In anotherinstance, the frame voids were filled with copper. In these instances,the electroplating was performed in a bath containing 125 g/lCuSO₄.5H₂O, 50 g/l H₂SO₄. The remaining frame precursor was removed bysoaking in N-methylpyrrolidone. The copper or nickel that remainedserved as the cores for the struts in the scaffold. In some instances, amaterial that serves as an active medium for a battery electrode iscoated onto the cores.

Example 4

Multiple different scaffolds were generated using the method of FIG. 6Athrough FIG. 6H. A thin layer of Cu was deposited onto the frame membersusing RF magnetron sputtering to improve electrical conductivity acrossthe nano-lattice. A conformal layer of amorphous Si with thickness ofhundreds of nanometers is sputtered over the Cu as the electroactivematerial. The frame members were then removed. The porosity of thestructure was selected such that expanding Si will fill a substantialfraction of free space in the structure upon lithiation, which resultsin high energy density. One of the scaffolds was built with octet unitcells and another scaffold was built with auxectic structures.

An electrochemical cell was built inside of a scanning electronmicroscope (SEM) using a lithium metal electrode, Li₂O solid electrolyteand the above scaffold. The Li metal electrode and the Li₂O electrolytewas mounted on a telescoping mechanical arm that extends into the SEMvacuum chamber. The above scaffolds were used as a Si electrode that wasplaced on the sample stage, and oriented so that changes in volume canbe observed using the scanning electron beam. The electrolyte wascontacted to the Si electrode and a constant voltage bias is appliedbetween the two electrodes during electrochemical cycling. Lithiation isperformed at a −4V bias, and delithiation is performed at a +4V bias.

The lithiation and delithiation of the octet structure showed thatlithiation causes each strut to change volume, and the overall structureto bow out slightly at locations of high lithiation. The auxecticstructure also showed volume change in each strut, but greater globalvolume change than the octet structure because expanding struts exertforce on each other and push adjoining unit cells away from each other.Accordingly, the Si electrode is suitable for use in secondarybatteries.

Example 5

Multiple different scaffolds were generated using the method of FIG. 6Athrough FIG. 6H. The shell was a thin film of alumina deposited onto theframe members using atomic layer deposition. After the deposition ofalumina, the original polymer scaffold was exposed to air via focusedion beam (FIB) milling and etched away in O₂ plasma. The resultingscaffold was a freestanding ceramic nanoscaffold comprised of periodichollow elliptical struts. The different scaffolds were generated so asto have a shell thickness varying from 5 to 60 nm, a major axis of 0.43to 1.32 micron, and octet unit cells with a width of 5-15 micron. Thescaffolds were fabricated with relative densities spanning from 0.21% to8.6%. Taking the density of solid ALD alumina to be ρ_(s)=2900 mg/cm³gives structural densities of ρ=6.1 to 249 mg/cm³, which places thelightest scaffolds in the ultralight regime (mg/cm³).

Monotonic and cyclical compression tests were performed in a G200Nanoidenter (Agilent Technologies). In a first set of experiments,scaffolds were compressed uniaxially to ˜50% strain at a rate of 10⁻³s⁻¹ to determine their yield stress and overall deformationcharacteristics. In a second set of experiments, structures werecyclically loaded and unloaded 3 times to ˜70% of their failure load,and the unloading slope of each of the cycles was measured and averagedto determine the Young's modulus. The unloading modulus was taken fromcyclic loading tests in order to mitigate the effect of loadingimperfections in the uniaxial tests. Additional samples were compressedin an in-situ nanomechanical instrument (Nanomechanics Inc.) to revealthe failure modes (yielding or buckling) that occurred duringdeformation.

Scaffolds having a structural ratio of the shell wall thickness to majoraxis length less than or equal to 0.02 did not exhibit catastrophicfailure or discrete strain bursts. Instead, the structures underwent aductile-like, controlled deformation, and the stresses fluctuated aroundthe peak stress after yielding. As the structural ratio of thestructures is decreased, the scaffolds exhibit a smoother and continuousdeformation. Surprisingly, these ceramic scaffolds recovered by up to98% after being compressed to 50% strain, and by ˜80% after compressionto 85% strain.

Although the above methods of scaffold fabrication are disclosed in thecontext of an octahedral scaffold, these methods are easily adapted toother unit cell types by using direct laser writing to define thedesired unit cell configuration.

Other embodiments, combinations and modifications of this invention willoccur readily to those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawings.

The invention claimed is:
 1. A method of fabricating a scaffold,comprising: employing multiphoton absorption to define frame members ina frame precursor; and using the frame members to form struts thatintersect at nodes so as to define repeating unit cells of a scaffold;and forming one or more shell layers on the frame members.
 2. The methodof claim 1, wherein using the frame members to form the struts includereplacing the frame members with cores of the struts.
 3. The method ofclaim 2, wherein the cores of the struts are a liquid or a gas.
 4. Themethod of claim 3; wherein replacing frame members with the coresincludes removing the frame members from within the one or more shelllayers.
 5. The method of claim 4, wherein the one or more shell layersincludes a ceramic.
 6. The method of claim 3, wherein replacing framemembers with the cores includes removing the frame members from withinthe frame precursor so as to form voids in the frame precursor andplacing in the voids a solid material that serves as the cores.
 7. Themethod of claim 3, further comprising: separating a portion of the frameprecursor that is not converted to the frame members from the cores. 8.The method of claim 1, wherein the frame precursor is sensitive tomultiphoton polymerization.
 9. A method of fabricating a micro- ornano-scale scaffold, comprising: providing a framework precursor,wherein the framework precursor is subject to multiphotonpolymerization; contacting the framework precursor with multiphotons ina desired configuration to generate a plurality of polymerized strutsinteracting at nodes thereby generating the micro- or nano-scalescaffold; and coating the polymerized struts with a material.
 10. Themethod of claim 9, wherein the scaffold is less than 50 microns acrossin a particular direction.
 11. The method of claim 9, wherein themultiphotons are provided by at least two lasers having a focal point ata desired location to polymerize a framework precursor.